In certain papermaking applications, a press nip on a papermaking machine is employed wherein the formed paper sheet contacts two dissimilar surfaces; the press fabric and one of the press rolls. Such press nips have, in the past, included a pair of counter-rotating steel rolls or a steel roll cooperating with a counter-rotating granite roll or the like.
Because the roll is smooth, no pattern or change in the paper product topography is caused during contact with this roll. The roll may also have some other function such as promoting paper smoothness as in a calender. The roll may also be a press roll. With a press roll, the sheet contacts the roll surface for some distance before the press nip that is formed by two rolls and a dewatering medium such as a press fabric. The paper sheet also contacts the roll surface for some distance after leaving the press nip. This arrangement not only enhances paper smoothness, but allows the sheet to be readily pulled off the roll surface easily and uniformly. If too much force is needed to cause separation, the wet paper sheet is drawn too much, loses MD stretch, and narrows in. In extreme situations the paper sheet will tear or break if the applied force is greater than the fiber-to-fiber bonds of the cellulose fibers that make up the paper. This causes loss of paper production and increases costs.
In certain applications, it has been found advantageous to cover the roll of the press nip with a cover for improving the surface characteristics of the resultant pressed web. Although various rubber compositions have been used in the fabrication of roll covers, polymeric materials have been used very successfully in the manufacture of such roll covers. The aforementioned polymeric materials may be reinforced with woven or non-woven fabric and may comprise single or multiple layers.
Roll covers are manufactured in a variety of ways, using varied materials such as rubber-like materials or metal alloys. Current methods of manufacturing roll covers include “laying up” sections of uncured mats of compounds onto a roll body, sometimes with textile reinforcement. This forms a roll cover of several sections, which is finished by heat curing to form a continuous cover. This cover is ground and may undergo other surface finishing steps. Another method used to form roll covers is to spiral wind a semi-solid resin onto a body, followed by curing and surface finishing. Steel strips may also be spiral wound onto the roll base in order to form a roll cover.
Yet another method is to cast or mold a resin-fiber system onto a mandrel or roll body, forming a composite system. Other covers may be formed of mixtures of resins (i.e., “alloys” of resins), metals and resins, ceramics, and the like. Roll covers can be applied with thermal coating techniques, such as arc spraying, powdered flame process, high velocity oxygen fuel (HVOF), and plasma spraying. (See e.g. “Beloit Partner” brochure, p. 25 (1995). Arc spraying primarily is used to coat or “clad” surfaces with metal or metals. Arc spraying parameters can be altered to provide desired surface properties such as hardness, porosity, thermal conductivity and bond strength. Powdered flame-spray processes are an economical alternative to plasma sprayed coatings and can be used where above-average resistance to corrosion and wear is desired. It is useful in applying a release coating. Plasma-coating processes use high temperatures and moderate spray velocities to produce a wide range of coatings. Yankeee Dryers, winder drums and after dryer shells are excellent applications for this process. HVOF-based coatings are applied in a process using high spray velocities to produce coatings with unusual properties. HVOF spraying is extremely dense, which means the coated finish can be ground to a fine grade smoothness. HVOF advances the ability to produce ceramet (ceramic and metallic) coatings, which combine the hardness and low porosity of ceramics with the flexibility of metals.
In this technique, oxygen and fuel are mixed in an HVOF spray gun. Once combined, they ignite to produce a supersonic gas jet traveling at speeds up to 4830 Km/h with an approximate temperature of 2816° C. A level of surface smoothness that can be attained with this process is greater than any other type of thermal spraying or metal manufacturing methods. In addition, HVOF coatings can assume different properties by spraying multicomponent powders. The bond strengths of HVOF coatings are also excellent.
Because the rolls used in papermaking process are both extremely heavy and very long, they tend to sag in the middle of the roll. Those skilled in the art have compensated for this by building up a crown in the center region of the roll., where the roll diameter may be up to 6 to 8 inches greater than the specified roll diameter. Furthermore, to insure an even load distribution across the length of the roll, the ends of the roll are tapered. Typically, the crown and the tapered end features are produced after the roll cover has been applied, usually by grinding and other surface treatments.
It would be advantageous to develop a method of forming a roll cover in which the differences in diameter across the length of the roll are provided for during application of the roll covering material, thereby eliminating or significantly reducing post processing steps that occur after the material has been applied, such as grinding.
Current release rolls include naturally occurring granite. Granite exhibits very good sheet release properties, resistance to chemical attack, resistance to wear from doctors, and can run for long intervals between grinds. However, it cannot run under high nip loading, is temperature sensitive, and can only be reground a limited number of times before the entire roll has to be replaced. Due to naturally occurring faults in granite, and the dangers of catastrophic failure, roll covers today have composite or alloy covers with a heterogeneous mixture of materials that are formed into a cover. Due to its heterogeneous nature, it allows the sheet to be released with forces low enough to avoid tearing the sheet. Expensive ceramics are also employed to form good release cover rolls. This mixing and the method of application assumes a relatively uniform dispersion of materials. This is not predictable however, and no two roll covers of the same mixture are therefore “exactly” alike.
Many of these covers have active (i.e., functional) surfaces that fulfill a desirable attribute, such as improving sheet release. See, for example, U.S. Pat. No. 6,136,157. Sheet release is a property of considerable importance to the paper maker, since there is a tendency for the wet paper sheet to follow the smoothest or wettest surface out of the press nip, which ordinarily is the roll cover. The wet paper sheet will follow the roll cover for a distance equivalent to part of the circumference of the roll cover as the sheet exits the nip. The sheet is literally pulled off the surface of the roll because the next downstream position is running at a higher speed. As speeds increase, the forces required to do this become substantial.
When the roll cover must provide a desirable attribute such as sheet release, it is important that it be applied uniformly. It should be understood that in at least some of the aforenoted techniques, application may not be uniform so that finishing steps such as surface grinding must be undertaken.
Where a press nip is arranged so that the formed paper sheet contacts two dissimilar surfaces, i.e., the press fabric and one of the press rolls, the surface characteristics of the roll cover can be used to emboss the formed paper sheet. That is, a pattern can be formed on the roll cover, with the pattern height, width, and depth dimensions which, when in contact with the formed paper sheet at high pressure, imparts a pattern to the paper sheet. Often in a press nip, the roll or rolls are patterned or vented to aid in water management. In this regard the outer face of the roll may be drilled or grooved in order to assist in channeling water away from the press nip. Additionally, roll covers have been used in the calendering of a web downstream from the press.
Methods of forming patterns on surfaces are described in the prior art. For example, U.S. Pat. No. 4,312,009 describes a device for projecting ink droplets onto a medium to print a pattern. A plate with holes is used to create the pattern on the medium. The pattern is determined by the arrangement of the holes on the plate.
U.S. Pat. No. 4,383,495 describes an apparatus for coating surfaces of a substrate. Fingers are inserted into holes in the substrate and ink is applied to the substrate surface. Upon removal of the fingers from the substrate, the ink is drawn into the substrate coating and walls.
U.S. Pat. No. 5,136,515 describes a three-dimensional article produced by layering droplets of two different materials. The first material forms the article. The second material provides support which is later removed.
U.S. Pat. No. 5,733,608 describes a system for coating a substrate. The substrate moves through a coating station where a coating and a carrying layer are applied to the substrate. The carrying layer is then removed leaving only the coating layer.
U.S. Pat. No. 5,817,374 describes a process for producing patterned articles by deposition of particles onto a surface. Particles are deposited onto the surface through a screen, mask or stencil. The arrangement of the holes in the screen, mask or stencil determines the resulting pattern.